A thermonuclear supernova interacting with hydrogen- and helium-deficient circumstellar material. SN 2020aeuh as a SN Ia-CSM-C/O?

TL;DR

SN 2020aeuh is a Type Ia-CSM with delayed interaction powered by a hydrogen- and helium-deficient CSM. The study combines extensive optical photometry and spectroscopy with pseudobolometric light-curve analysis and a CSM-interaction model to reveal a two-phase evolution: an early SN Ia-like peak followed by a long, CSI-driven rebrightening, with $M_{ m CSM} \approx 1$--$2\,M_{\odot}$ and a nickel mass of $M_{ m Ni} \approx 0.86\,M_{\odot}$. Late-time spectra lack narrow H or He lines but show potential narrow C II and O I features, highlighting a C/O-rich CSM; the host is a massive, quiescent galaxy, unusual for Ia-CSM hosts. The results challenge standard single-degenerate and double-degenerate progenitor scenarios, suggesting the need for alternative mass-loss or interaction channels to explain Ia-CSM-C/O events.

Abstract

Identifying the progenitors of thermonuclear supernovae (Type Ia supernovae; SNe Ia) remains a key objective in contemporary astronomy. The rare subclass of SNe Ia that interacts with circumstellar material (Type Ia-CSM) allows for studies of the progenitor's environment before explosion, and generally favours single-degenerate progenitor channels. The case of SN Ia-CSM PTF11kx clearly connected thermonuclear explosions with hydrogen-rich CSM-interacting events, and the more recent SN 2020eyj connected SNe Ia with helum-rich companion progenitors. Here we present a study of SN 2020aeuh, a Type Ia-CSM with delayed interaction. We analyse photometric and spectroscopic data that monitor the evolution of SN 2020aeuh and compare its properties with those of peculiar SNe Ia and core-collapse SNe. At early times, the evolution of SN 2020aeuh resembles a slightly overluminous SN Ia. Later, the interaction-dominated spectra develop the same pseudocontinuum seen in Type Ia-CSM PTF11kx and SN 2020eyj. However, the later-time spectra of SN 2020aeuh lack hydrogen and helium narrow lines. Instead, a few narrow lines could be attributed to carbon and oxygen. We fit the pseudobolometric light curve with a CSM-interaction mode, yielding a CSM mass of 1-2 M$_{\odot}$. We propose that SN 2020aeuh was a Type Ia supernova that eventually interacted with a dense medium which was deficient in both hydrogen and helium. Whereas previous SNe Ia-CSM constitute our best evidence for nondegenerate companion progenitors, the CSM around SN 2020aeuh is more difficult to understand. We include a hydrodynamical simulation for a double-degenerate system to showcase how the dynamical evolution of such a progenitor scenario could produce the CSM observed around SN 2020aeuh. It is clear that SN 2020aeuh challenges current models for stellar evolution leading up to a SN Ia explosion.

Figures (9)

• Figure 1: The field of SN 2020aeuh and its host galaxy. The colour image of the field is constructed from Pan-STARRS images in the gri bands chambers2016. The inset shows a negative image of the host galaxy, taken with P60 in the g-band filter at an epoch of 197.8 days. The position of SN 2020aeuh is indicated with red arrows.
• Figure 2: Light curves of SN 2020aeuh, corrected for MW extinction, with phases in rest-frame days since first detection. Early nondetections with 5$\sigma$ upper limits are indicated by downward-facing empty triangles. Apparent magnitudes are provided on the left-hand axis and absolute magnitudes to the right. The photometry has been binned into one-night bins. The arrows on top indicate the epochs of spectroscopy. The lower panel shows the $g - r$ colour evolution.
• Figure 3: Comparison of the classification spectrum of SN 2020aeuh (in black) with a variety of SNe Ia from the literature (see text for discussion and references). The locations of some common transitions are provided at the bottom.
• Figure 4: Spectral sequence of SN 2020aeuh at late phases displaying our 7 spectra between 81 and 181 days past first detection, when the SN is clearly dominated by CSM interaction. The smoothed spectra (using a Savitzky–Golay filter) are shown in solid lines while the original in semi-transparent. The spectral evolution is rather slow during these 100 days. The spectrum with best signal, at 99 days, is compared to spectra of other SNe in Fig. \ref{['fig:csm_spectrum_comparison']}.
• Figure 5: A comparison of the spectrum of SN 2020aeuh at +99 days with a selection of other interacting Type I SNe. The shown Type Ia-CSM (in red) are SNe 2018evt, 2020aekp, 2020qxz Sharma2023, and SN 2020eyj Kool2022. Type Ibn (blue): SN 2006jc foley2007, SN 2019deh, and SN 2019iep. Type Ic (brown): SN 2019hgp Galyam2021 and SN 2021csp perley2022. All spectra have been shifted and multiplied by a numerical factor for visual purposes. Phases are relative to first detection. Dashed lines mark the calcium features. Solid lines indicate the calcium velocity offset estimated for SN 2020aeuh.